A force sensing resistor (FSR) sensor, also known as a FSR, a force sensor, or a pressure sensor, is a type of variable resistor whose resistance decreases when the applied force increases. If a large enough force is applied, a short circuit is formed enabling electric current flow. Additional force further decreases the resistance of the force sensitive resistor and provides corresponding lower resistance for the electric current. In contrast, if too small or no force is applied, the resistance remains and provides high resistance to the electric current. An amount of sensed current through the FSR sensor can be used to determine an amount of applied force. A typical FSR sensor includes a circuit having a first conductive layer, typically a conductive film, and a second conductive layer. In a static state, the first conductive layer and the second conductive layer are separated from each other by an air gap. This separation between the two conductive layers forms an open circuit, and no current flows through the circuit. When force is applied to one or both of the first and second conductive layers, the first and second conductive layers are forced toward each other. If sufficient force is applied, the first and second conductive layers contact each other causing a short circuit that enables current flow through the circuit. The circuit is sensed for current to determine a size of the force applied to the FSR sensor.
FIG. 1 illustrates an exploded view of a conventional FSR sensor. The conventional FSR sensor includes a first conductive layer 4 formed on a first substrate 2, a second conductive layer formed on a substrate 12, and a spacer 6 that separates the first conductive layer 4 and the second conductive layer. The second conductive layer is made of two patterned conductive traces 14 where the two patterned conductive traces are separated from each other to form an open circuit. Each patterned conductive trace 14 has a terminal extension 16 that provides an external connection to the FSR. The external connection is connected to a current source and a current sensing circuit. The spacer 6 includes an opening 8, which forms an air cavity between the first conductive layer 4 and the patterned conductive traces 14. The spacer 6 also includes an air vent 10 connected to the opening 8, which forms an air vent channel between the substrate 2 and the substrate 12 along the length of the terminal extensions 16. In an assembled, static state, the conductive layer 4 is separated from the patterned conductive traces 14 by the air gap in the opening 8. When a force is applied to either the substrate 2, the substrate 12, or both, the first conductive layer 4 and the conductive traces 14 are forced toward each other. If the applied force is great enough, some or all of the first conductive layer contacts some or all of the patterned conductive traces 14. In this contacted state, the first conductive layer 4 forms a shunt between the two separated patterned conductive traces 14, forming a short circuit. With the short circuit engaged, current flows from one of the patterned conductive traces 14 to the other, via the first conductive layer 4, and is sensed by the current sensing circuit indicating the presence of the applied force.
To enable the first conductive layer 6 and the patterned conductive traces 14 to move toward each other under applied force, the air in the cavity formed by the opening 8 must be vented out of the air vent 10. When the applied force is removed the first conductive layer 4 and the pattered conductive traces 14 move back to their static state positions, air returns into the cavity through the air vent 10. However, the air vented in and out of the air cavity in the opening 8 is vented to the environment, which allows dust and moister to enter into the FSR and in particular in between the first conductive layer 4 and the patterned conductive traces 14. Such contaminant ingress into the FSR may cause irregularities and interfere with the measurements.